56 Professors Ayr ton and Perry on the Seat 



distinct method of looking at the matter, has been misappre- 

 hended by Dr. Lodge. 



n L Zi. 



H 



Suppose we have a circuit of pipes, G A B D D' W E G, 

 whose development is shown in the figure, the two ends of 

 the figure G and G being the same point in the real circuit ; 

 the pipes to be of uniform section, and to rise suddenly in 

 level from A to B and from D to D', and to fall from E ; to 

 E, and there being a uniform rate of fall from B to D, D' to 

 W, and from E through G to A. HI may be any datum- 

 level. Let us suppose that there is a continuously acting 

 pump in the part D' E', causing incompressible fluid, such 

 as water, filling the pipes to flow continuously. Mechanical 

 energy is given to the pump from some outside source, and 

 the energy reappears at various places in the circuit. Thus, 

 for example, between A and B the water rises in level. A 

 pound of water at B has more energy than a pound of water 

 at A, and its increase of energy is h foot-pounds, if the dif- 

 ference of level is h feet between B and A. It has really 

 gained energy. What has it lost ? Pressure, certainly ; but 

 pressure is not a form of energy. We know that in a steady 

 frictionless stream 



remains constant, if v is the velocity in feet per second, and 

 p the pressure in pounds per square inch, and h height in feet 



above datum-level. 



As k- is the kinetic energy of a pound 



of water in foot pounds, and h is its potential-energy, one is 

 tempted to call 2*3 p also a form of energy — pressure-energy ; 

 but we have just as much and no more right to call it so, 

 than to speak, as we did, of tension-energy in electricity. We 

 can see that pressure conditions are not in themselves a 

 source of energy, by imagining incompressible fluid, like 

 water, filling a closed vessel at great pressure, and then 

 imagining an orifice to be made, when, of course, the water 



